1. Field of the Invention
This invention relates to a solenoid driver circuit and, more particularly, to a solenoid driver circuit designed to correct variations in the characteristics of a solenoid due to temperature.
2. Description of the Related Art
The value of the electrical resistance of a solenoid varies in proportion to the temperature of the winding. If the winding temperature rises in accordance with the energization time and the energization current value, the electrical resistance increases. When the electrical resistance increases, the current flowing through the solenoid drops even if the same voltage is applied. As a result, the attracting force decreases, resulting in a decrease in the operating speed of the actuator. Where the temperature is lower than the specified temperature, overcurrent will flow through the solenoid, and settling to the target current value will be delayed in the same way as in cases of high temperatures. Depending on the mode of use of the solenoid actuator, variations in the attracting force due to the temperature of the winding may present no problems. However, where more accurate control of attracting force is necessary, variations in the attracting force pose problems.
FIG. 3 shows the current through a solenoid in a cutting machine for cut sheet that makes cut lettering by driving a cutting head up and down by a solenoid actuator. A voltage is supplied from a driver circuit in a given pattern, whereby the solenoid is activated. At this time, the current value varies according to the temperature of the solenoid. As the temperature rises, the current value drops and lowers the landing speed of the cutting head. This may make the cutting depth uneven when the head lands and at the beginning of the cutting. Furthermore, the deviation from the voltage necessary to match the target current value becomes greater than a prescribed value and increases the amount of feedback control. Time differences (S1, S2, and S3) occur until the current is controlled to a certain current. If one tries to cover the used temperature range, he or she must wait until Smax. This leads to a decrease in the cutting speed.
Accordingly, a technical problem to be solved occurs to eliminate variations in the operating characteristics due to the temperature of the winding of a solenoid. The present invention is intended to solve the foregoing problem.
This invention is proposed to achieve the above-described object, and provides a solenoid driver circuit including: control means for applying a voltage less than an activation threshold value to a solenoid from a power supply when the solenoid is deactivated; means for detecting a current value when the voltage is applied; means for computing a resistance value of the solenoid with the applied voltage and the detected current value; and constant-current control means for maintaining the current value constant during operation by controlling a solenoid driving voltage according to the calculated resistance value.